In an address to the United Nations General Assembly, Ri Yong Ho said the use of nuclear weapons is not Pyongyang's "first option." But he also cautioned that the U.S. and its allies should "think twice" before threatening North Korea.

The comments came moments after the U.S. flew B-1B bombers and F-15 fighter jets in international airspace east of North Korea — the farthest north of the Demilitarized Zone (DMZ) between North and South Korea that U.S. fighters or bombers have ever flown.

The Pentagon said the flight was intended as a signal to North Korea that the U.S. does, in fact, have military options in place for responding to potential threats from Pyongyang.

North Korea has stepped up weapons tests over the past year. Earlier this month, it detonated for the first time what North Korean officials claimed was a hydrogen bomb — a development that would signal a major milestone for the country's nuclear weapons program.

Ri said Saturday that North Korea has nearly completed its goal of developing a workable nuclear arsenal. He defended the country's nuclear program as a necessary deterrent against U.S. aggression.

"The very reason [North Korea] had to possess nuclear weapons is because of the U.S.," he said.

Trump and Kim have been engaged for months in an ongoing war of words and threats.

In a speech before the U.N. earlier this week, Trump called Kim a "Rocket Man on a suicide mission," and threatened to "totally destroy" the North if the U.S. is forced to take military action.

The North Korean leader responded to Trump's speech in a rare statement on Friday, calling Trump "mentally deranged" and claiming that he would strike the U.S. when he deemed necessary.

"I will surely and definitely tame the mentally deranged U.S. dotard with fire," Kim said.